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  Java Practices -> Consider composition instead of subclassing
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<div class='page-title'>Consider composition instead of subclassing</div>

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<br>Consider composition as an alternative to subclassing.
<p>Composition :
<ul>
<li>
is implemented simply by forwarding all calls to an object field</li>

<li>
has no dependence on implementation details of the object field</li>

<li>
is more flexible, since it is defined dynamically at run-time, not statically
at compile-time</li>
</ul>
Subclassing has the following issues :
<ul>
<li>
it violates encapsulation, since the implementations of the superclass
and subclass become tightly coupled</li>

<li>
new methods added to the superclass can break the subclass</li>

<li>
superclass and subclass need to evolve in parallel</li>

<li>
designing a class so that it may be safely extended takes extra work -&nbsp;
extending a class not explicitly designed for such use is risky</li>

<li>
different packages are often under the control of different programmers
- extending a class in a different package is risky</li>
</ul>
In addition :
<ul>
<li>
<tt>Serializable</tt> and <tt>Cloneable</tt> are more difficult to implement
when inheritance is present</li>

<li>
given a concrete class <tt>Aircraft</tt> which implements <tt>Comparable</tt>
and overrides <tt>equals</tt>, it is not possible to extend such a class
(<tt>Boeing747</tt>) and add a new significant field (<tt>fUpperDeck</tt>),
<i>and</i>
simultaneously provide completely correct implementations of <tt>compareTo</tt>
and
<tt>equals</tt>. In this situation, composition is required. (See 
<a href='http://www.amazon.com/exec/obidos/ASIN/0321356683/ref=nosim/javapractices-20'><i>Effective Java</i></a> 
for further discussion.)</li>
</ul>

<P>A common exception is the <a href="TopicActione01b-2.html">template method</a> design pattern. 
There, the safest style is to make all items in the abstract base class <tt>final</tt>, except for the single 
<tt>abstract</tt> method which needs to be implemented by the subclass.

<P>An interesting quote from chapter one of <i>Design Patterns</i> :
<p>"<i>That leads us to our second principle of object-oriented design
:</i>
<p><i>Favor object composition over class inheritance.</i>
<p><i>...Nevertheless, our experience is that designers overuse inheritance
as a reuse technique, and designs are often made more reusable (and simpler)
by depending more on object composition. You'll see object composition
applied again and again in the design patterns.</i>"
<p>(Their first principle, stated earlier in the chapter, is "<i>Program
to an interface, not an implementation</i>".)
<br>
<br>

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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction3eed-2.html?Id=10'>Implementing compareTo</a> <br>
 
  
  <a href='TopicAction1deb-2.html?Id=17'>Implementing equals</a> <br>
 
  
  <a href='TopicAction23a3-2.html?Id=123'>Minimize ripple effects</a> <br>
 
  
  <a href='TopicActione01b-2.html?Id=164'>Template method</a> <br>
 
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